1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly relates to a semiconductor device in which bridge circuits are made up by switching elements mounted on die pads.
2. Description of the Related Art
Semiconductor devices for driving three-phase motors are in wide use in order to control operations of motors which are used for not only home electric applications and industrial electric applications but also for compressors of air conditioning units, drums of washing machines, and pumps so on. Patent Document 1 (Laid-Open Patent Publication No. 2003-348874) discloses a drive circuit of a semiconductor device for driving a three-phase motor with reference to FIG. 3. In the drive circuit, three half-bridge circuits in which high side (H-side) switching elements and low side (L-side) switching elements are connected in series are prepared for a phase U, a phase V, and a phase W. Further, the three half-bridge circuits are electrically connected in parallel. In short, the drive circuit includes a total of six switching elements. The high-side switching elements have their drain electrodes electrically connected to one end of a DC power source (having a fixed potential at a high voltage side), and have their source electrodes electrically connected to output terminals for the phases U, V and W in parallel. The low-side switching elements have their drain electrodes electrically connected to output terminals for the phases U, V and W in parallel, and have their source electrodes electrically connected to the other end of the DC power source (having a fixed potential at a low voltage side) in parallel.
Patent Document 2 (Laid-Open Patent Publication No. 2009-130055) discloses a specific structure of a semiconductor device which is suitable to driving a three-phase motor. The semiconductor device includes not only a vertical power MOSFET (metal oxide semiconductor field elect transistor) as a high-side switching element on one drain lead but also another vertical power MOSFET as a low-side switching element on the other drain lead. The high-side power MOSFET has a drain electrode on a rear surface of a semiconductor chip. The drain electrode is connected to one end of the DC power source via a drain lead. Further, the high-side power MOSFET includes a source electrode on a front surface of the semiconductor chip. The source electrode is connected not only to an output terminal via a wire and a source lead but also to a low-side drain lead. On the other hand, the low-side power MOSFET has a drain electrode on the rear surface of the semiconductor chip. The drain electrode is connected to the output terminal via a drain lead. Further, the low-side power MOSFET has a source electrode on the front surface of the semiconductor chip. The source electrode is connected to the other end of the DC power source via a wire and a source lead.
The semiconductor device of the Patent Document 2 seems to have the following problems. The drain lead is installed on a heat sink, and effectively radiates heat generated in response to the operation of the power MOSFET. One end of the DC power source is connected to the high-side drain lead, to which DC power having a fixed potential is applied. This is effective in enlarging the drain lead and improving the radiation performance.
On the contrary, a low-side drain lead is connected to the output terminal, and is applied with DC power having a fluctuating potential. If the drain lead is enlarged in order to improve the radiation performance, noises or parasitic capacitance tends to be increased. Increase of noises would raise the event provability of false operations while increase of parasitic capacitance slows down the operation speed. When the drain lead of the low-side power MOSFET is insulated from the heat sink, it is possible to suppress influence of noises. However, when insulated, more heat is generated by the power MOSFET, and the radiation performance of the heat sink cannot be raised due to thermal resistance.